A printed wiring board is generally produced by attaching an insulating substrate to a copper foil to form a copper clad laminate and then forming a conductor pattern on the copper foil surface by etching. Component mounting density and signal frequency have been increased, with the reduction in size and increase in need of high performance of recent electronic devices, and printed wiring boards have been required to have highly fine (fine pitch) conductor patterns and to cope with high frequencies.
Recently, the copper foil having a thickness of 9 μm or less, even 5 μm or less is demanded in accordance with fine pitches. Such an ultra-thin copper foil has low mechanical strength to readily cause tear or wrinkles during production of printed circuit boards. Thus, a copper foil with a carrier composed of a thick metal foil as a carrier and an ultra-thin copper layer electrodeposited onto the metal foil with a release layer therebetween has been developed. The copper foil with a carrier is usually used by bonding the surface of the ultra-thin copper layer to an insulating substrate by thermocompression bonding and then peeling the carrier with the release layer.
The surface of the ultra-thin copper layer of the copper foil with a carrier, i.e., the surface to be bonded to a resin, is primarily required to have a sufficient peel strength between the ultra-thin copper layer and the resin base material and to sufficiently maintain the peel strength after heating to high temperature, wet processing, soldering, chemical treatment, and other treatments.
In general, the peel strength between an ultra-thin copper layer and a resin base material is typically enhanced by allowing a large amount of roughening particles to adhere to the ultra-thin copper layer to increase the profile such as unevenness and roughness of the surface.
However, if such an ultra-thin copper layer having such an increased profile such as unevenness and roughness is applied to a semiconductor package substrate, which is a printed wiring board being particularly required to have a fine circuit pattern, unnecessary copper particles remain during circuit etching to cause defects such as insulation failure between circuit patterns.
Accordingly, a copper foil with a carrier prepared without performing roughening treatment of the ultra-thin copper layer surface has been used as the copper foil with a carrier for fine circuits such as a semiconductor package substrate. The adhesion (peel strength) to a resin of such an ultra-thin copper layer not subjected to roughening treatment is apt to decrease compared to a copper foil for a general printed wiring board due to the low profile such as unevenness, degree of roughness and roughness (see Patent Literature 8). Thus, the copper foil with a carrier needs further improvement.
The copper foil for a semiconductor package substrate is also generally referred to as a copper foil for a printed wiring board and is usually produced by the following procedure. First, a copper foil is laminated and bonded to a base material such as a synthetic resin under high-temperature and high-pressure. Next, in order to form an intended electrically conductive circuit on a substrate, a circuit corresponding to the intended circuit is printed on the copper foil with a material such as an etching resistant resin.
The unnecessary portion of the exposed copper foil is then removed by etching. After the etching, the printed portion of the materials such as the resin is removed to form an electrically conductive circuit on the substrate. The formed electrically conductive circuit is finally formed into a variety of printed circuit boards for electronic devices by soldering specified elements.
Finally, the resulting circuit board is joined to a resist or build-up resin substrate. In general, the quality requirements for a copper foil for a printed wiring board differ between the bonding surface (i.e., roughened surface) to be bonded to a resin base material and the non-bonding surface (i.e., glossy surface). Such different requirements have to be simultaneously satisfied.
The requirements for the glossy surface include (1) satisfactory appearance and no oxidative discoloration during storage, (2) satisfactory solder wettability, (3) no oxidative discoloration during high-temperature heating, and (4) satisfactory adhesion with a resist.
On the other hand, the requirements for the roughened surface mainly include (1) no oxidative discoloration during storage, (2) maintenance of sufficient peel strength with a base material after high-temperature heating, wet processing, soldering, chemical treatment, and other treatment, and (3) no laminate spots after laminating with a base material or etching.
In addition, recent finer patterns demand lower profiles of a copper foil; namely, an increase in peel strength of the roughened surface of a copper foil is necessary in accordance with it.
Furthermore, in electronic devices such as personal computers and mobile communication devices, as the speed and capacity of communication increase, the frequencies of electrical signals are increased. A printed wiring board and a copper foil that can cope with such progress are demanded. An electrical signal frequency of 1 GHz or more significantly increases the influence of a skin effect, the current flowing only on the surface of a conductor, to cause a change in current transmitting path due to the unevenness of the surface and to thereby increase the impedance to a level that is not negligible. From this point, the surface roughness of a copper foil is desired to be small.
In order to satisfy such a requirement, a variety of methods of treating a copper foil for a printed wiring board have been proposed.
In usual treatment of a copper foil for a printed wiring board, a rolled copper foil or an electrolyzed copper foil is used, and roughening treatment, in general, application of microparticles made of copper or copper oxide to the surface of the copper foil, is performed for increasing the adhesiveness (peel strength) between the copper foil and a resin. Then, in order to give properties of heat-resistant/rustproof, a heat-resistant layer, in another word, ‘a barrier layer’ of brass or zinc is formed.
In order to avoid surface oxidation or the like during transportation or storage, rust prevention treatment such as immersion or electrolytic chromate treatment or electrolytic chromium/zinc treatment is performed to yield a product.
Among these treatment processes, a roughened layer particularly has an important part in enhancement of the adhesiveness (peel strength) between a copper foil and a resin. Conventionally, roundish or spherical projections have been believed to be suitable for the roughening treatment. Such roundish projections are obtained by suppressing the development of dendrites. However, the roundish projections are detached at the time of etching, causing a phenomenon called “powder fall.” Since the contact area between the spherical projection and a copper foil is very small compared to the diameter of the roundish or spherical projection, the phenomenon inevitably occurs.
In order to avoid this “powder fall” phenomenon, a thin copper plating layer is formed on the projections after the roughening treatment to prevent the projections from peeling (see Patent Literature 1) off. This has an effect of preventing “powder fall”, but has disadvantages, that is, an increase in the number of steps and a variation in the effect of preventing “powder fall” depending on the thin copper plating.
It is also reported on a technology of forming an acicular nodular coating layer of an alloy of copper and nickel on a copper foil (Patent Literature 2). This nodular coating layer has projections in an acicular form and is thereby believed to show higher adhesion strength with a resin compared to the roundish or spherical projections disclosed in Patent Literature 1. The layer is made of a copper-nickel alloy, which is different from the component of the copper foil serving as the base, and is therefore etched at an etching rate different from that of forming a copper circuit. Consequently, such a layer is unsuitable for a stable circuit design.
In formation of a copper foil for a printed wiring board, a heat-resistant/rustproof layer is usually formed. As examples of the heat-resistant treatment layer of a metal or alloy, coating layers of Zn, Cu—Ni, Cu—Co, or Cu—Zn are applied to a large number of copper foil layers in practical use (e.g., see Patent Literature 3).
In particular, a copper foil provided with a heat-resistant treatment layer made of Cu—Zn (brass) has excellent characteristics such that lamination to a printed circuit board of, for example, an epoxy resin does not cause spots of the resin layer and that the peel strength is hardly decreased by high-temperature heating and is therefore widely used industrially.
Methods of forming the heat-resistant layer from brass are described in detail in Patent Literatures 4 and 5.
It has been proposed to improve the hydrochloric acid resistance by subjecting the surface of a copper foil to roughening treatment, rust prevention treatment with zinc or a zinc alloy, and chromate treatment and then adsorbing a silane coupling agent containing a small amount of chromium ions to the chromate-treated surface (see Patent Literature 7).